1. Field of the Invention
The present invention relates to an exhaust gas treatment catalyst for removal of one or more pollutants in an exhaust gas, an exhaust gas treatment method, and an exhaust gas treatment apparatus for removal of one or more pollutants in exhaust gas.
2. Description of the Related Art
An ammonia catalytic reduction method, in which NOX is decomposed into harmless nitrogen and water by ammonia (NH3) serving as a reducing agent in the presence of a nitrogen oxide removal catalyst (hereafter referred to as “a denitration catalyst”), has been in practical use as a method for removing nitrogen oxides (NOX) in exhaust gases discharged from boilers, gas turbines, incinerators, and the like.
In some of the above-described boilers and the like, coal, fuel oil C, or the like having a high sulfur content is used as a fuel. High concentrations of sulfur dioxide (SO2) and sulfur trioxide (SO3) are present in exhaust gases resulting from burning of such fuels.
When such an exhaust gas is treated by using the above-described ammonia catalytic reduction method, an oxidation reaction of SO2 to sulfur trioxide (SO3) occurs at the same time with a NOX reduction and removal reaction, in which NOX is reduced and removed, and the content of SO3 in the exhaust gas is increased. The resulting SO3 and unreacted NH3, which serves as a reducing agent in the above-described NOX reduction and removal reaction, are readily bonded to each other in a low temperature region so as to form compounds, e.g., acid ammonium sulfate. The insides and pipes of various apparatuses, e.g., heat exchangers, disposed downstream are corroded by the resulting compounds, e.g., acid ammonium sulfate, and SO3, and clogging, partial blocking, or the like occurs so as to increase the pressure loss.
Consequently, in the case where the above-described exhaust gas is treated, a titania-vanadium-tungsten catalyst or the like is used as a denitration catalyst having excellent denitration performance and low SO2 oxidation performance resistant to occurrence of oxidation reaction of SO2 to SO3.
On the other hand, examples of the above-described exhaust gas treatment catalysts include a catalyst 40, in which the entire catalyst is composed of a powder 41 having SO3 reduction performance, as shown in FIG. 4.
Furthermore, various technologies for reducing the concentration of the above-described sulfur trioxide (SO3) in the exhaust gases have been proposed (for example, Japanese Unexamined Patent Application Publication No. 10-249163, Japanese Unexamined Patent Application Publication No. 11-267459, and Japanese Unexamined Patent Application Publication No. 2006-136869).
However, it is known that the oxidation reaction of SO2 to SO3, on the order of 0.1%, occurs even when the above-described titania-vanadium-tungsten catalyst serving as the denitration catalyst is used. Therefore, acid ammonium sulfate and the like are generated, as described above.
Here, a reaction mechanism in the case where the concentration of NH3 is reduced in the above-described exhaust gas treatment catalyst 40 will be described with reference to FIG. 5. In the drawing, each line represents the concentration of one component in a thickness direction of the catalyst 40 perpendicular to the gas flow. A solid line represents the concentration of NH3, a dotted chain line represents the concentration of NOX, and a two-dot chain line represents the concentration of SO3.
As is clear from FIG. 5, the concentrations of NH3 and NOX are high on the surface of the catalyst 40, but are decreased with decreasing proximity to the surface so as to become constant. On the other hand, the concentration of SO3 is decreased with decreasing proximity to the surface of the catalyst 40 in the vicinity of the surface, but thereafter, is increased with decreasing proximity to the surface. That is, in the vicinity of the surface of the catalyst 40, a denitration reaction represented by the following formula (1), a SO3 reduction reaction represented by the following formula (2), and a self decomposition reaction of ammonia represented by the following formula (3) are facilitated. Furthermore, in the inside of the catalyst 40, a SO3 formation reaction represented by the following formula (4) is facilitated. Specifically, it was made clear that, in the vicinity of the surface of the catalyst 40, a denitration reaction region S1 and a SO3 reduction reaction region S2 were dominant, while an ammonia self decomposition region was dominant in the same region, whereas merely a SO3 formation reaction S3 was dominant in the inside of the catalyst 40.4NO+4NH3+O2→4N2+6H2O  (1)SO3+2NH3+O2→SO2+N2+3H2O  (2)4NH3+3O2→2N2+6H2O  (3)2SO2+O2→2SO3  (4)
It is believed that examples of structures suitable for inhibiting the formation of SO3 even when the NH3 concentration is reduced, as described above, include a catalyst 50, as shown in FIG. 6, in which a SO3-reducing catalyst portion 52 having the SO3 reduction performance is disposed on the surface of a base material 51, e.g., cordierite, and a catalyst 60, as shown in FIG. 7, in which a SO3-reducing catalyst portion 62 having the SO3 reduction performance is disposed on the surface of a denitration catalyst 61.
However, with respect to even the catalyst 50 as shown in FIG. 6, since the SO3-reducing catalyst portion 52 is disposed merely in the vicinity of the surface of the base material 51, in the case where an exhaust gas contains ash, the SO3-reducing catalyst portion 52 is abraded by the ash and, thereby, the catalytic performance thereof is degraded. Furthermore, in the case where an exhaust gas contains a poison component, e.g., arsenic, since the components are different between the base material 51 and the SO3-reducing catalyst portion 52, the poison component diffuses into merely the SO3-reducing catalyst portion 52 so as to poison merely the catalyst portion 52.
With respect to even the catalyst 60 as shown in FIG. 7, since the SO3-reducing catalyst portion 62 is disposed merely in the vicinity of the surface, in the case where an exhaust gas contains ash, the SO3-reducing catalyst portion 62 is abraded by the ash and, thereby, the catalytic performance thereof is degraded. Furthermore, in the case where an exhaust gas contains a poison component, e.g., arsenic, since the denitration catalyst 61 and the SO3-reducing catalyst portion 62 contain the same component, the poison component diffuses into the SO3-reducing catalyst portion 62 and the denitration catalyst 61 so as to poison the entire catalyst 60.
The above-described problems occur with respect to not only catalysts which facilitate a reduction reaction of sulfur trioxide and reduction reactions of nitrogen oxides, but also exhaust gas treatment catalysts, such as NOX-reducing catalysts and SOX-reducing catalysts, which remove one or more pollutants in an exhaust gas.
The present invention has been proposed in consideration of the above-described circumstances. Accordingly, it is an object of the invention to provide an exhaust gas treatment catalyst, an exhaust gas treatment method, and an exhaust gas treatment apparatus, in which the performance degradation due to abrasion and poisoning is suppressed.